Data retention indicator for magnetic memories

ABSTRACT

The present invention provides an array ( 20 ) of magnetoresistive memory elements ( 10 ) provided with at least one data retention indicator device ( 50 ). The at least one data retention indicator device ( 50 ) comprises a first magnetic element ( 51 ) and a second magnetic element ( 52 ) each having a pre-set magnetisation direction, the pre-set magnetisation direction of the first and second magnetic elements ( 51, 52 ) being different from each other. The first and second magnetic elements ( 51, 52 ) are suitable for aligning their magnetisation direction with magnetic field lines of an externally applied magnetic field exceeding a detection threshold value. According to the present invention, a parameter of the at least one data retention indicator device ( 50 ) is chosen so as to set the detection threshold value of the externally applied magnetic field to be detected. The at least one data retention indicator device ( 50 ) has a state or an output indicative of exposure of the magnetoresistive memory elements ( 10 ) of the array ( 20 ) to said externally applied magnetic field.

The present invention relates to magnetic memories, e.g.magnetoresistive random access memories (MRAM), and more particularly toa method and a device to indicate data retention of such magneticmemories, and thus to indicate error-free magnetic memory performance.

Magnetic or Magnetoresistive Random Access Memory (MRAM) is currentlybeing considered by many companies as a successor to flash memory. Ithas the potential to replace all but the fastest static RAM (SRAM)memories. This makes MRAM very suitable as embedded memory for System onChip (SoC). It is a non-volatile memory (NVM) device, which means thatno power is required to sustain the stored information. This is seen asan advantage over most other types of memory. MPAM memories can be usedin particular for ‘mobile’ applications, such as smartcards, mobilephones, PDA's, etc.

The MRAM concept was originally developed at Honeywell Corp. USA, anduses magnetisation direction in a magnetic multi-layer device asinformation storage and the resultant resistance difference forinformation readout. As with all memory devices, each cell in an MRAMarray must be able to store at least two binary states which representeither a “1” or a “0”.

Different kinds of magnetoresistive (MR) effects exist, of which theGiant Magneto-Resistance (GMR) and Tunnel Magneto-Resistance (TMR) arecurrently the most important ones. The GMR effect and the TMR effectprovide possibilities to realise a.o. non-volatile magnetic memories.These devices comprise a stack of thin films of which at least two areferromagnetic or ferrimagnetic, and which are separated by anon-magnetic interlayer. GMR is the magneto-resistance for structureswith conductor inter-layers and TMR is the magneto-resistance forstructures with dielectric inter-layers. If a very thin conductor isplaced between two ferromagnetic or ferrimagnetic films, then theeffective in-plane resistance of the composite multi-layer structure issmallest when the magnetisation directions of the films are parallel andlargest when the magnetisation directions of the films areanti-parallel. If a thin dielectric interlayer is placed between twoferromagnetic or ferrimagnetic films, tunneling current between thefilms is observed to be the largest (or thus resistance to be thesmallest) when the magnetisation directions of the films are paralleland tunneling current between the films is the smallest (or thusresistance the largest) when the magnetisation directions of the filmsare anti-parallel.

Magneto-resistance is usually measured as the percentage increase inresistance of the above structures going from parallel to anti-parallelmagnetisation states. TMR devices provide higher percentagemagneto-resistance than GMR structures, and thus have the potential forhigher signals and higher speed. Recent results indicate tunnelinggiving over 40% magneto-resistance, compared to 10-14%magneto-resistance in good GMR cells.

A typical MRAM device comprises a plurality of magnetoresistive memoryelements 10 of which one is illustrated in FIG. 1, e.g. magnetictunneling junction (MTJ) elements, arranged in an array. An array 20 ofmagnetoresistive memory elements 10 is illustrated in FIG. 2.Magnetoresistive memory elements 10 generally include a layeredstructure comprising a fixed or pinned hard magnetic layer 11, a freelayer 12 and a dielectric barrier 13 in between. The pinned layer 11 ofmagnetic material has a magnetic vector that always points in the samedirection. The free layer 12 is used for information storage. Themagnetic vector of the free layer 12 is free, but constrained within theeasy axis of the layer 12, which is determined chiefly by the physicaldimensions of the magnetoresistive memory element 10. The magneticvector of the free layer 12 points in either of two directions: parallelor anti-parallel with the magnetisation direction of the pinned layer11, which coincides with the said easy axis. The fundamental principleof MRAM is the storage of information as binary data, e.g. as “0” and“1”, based on directions of magnetisation. This is why the magnetic datais non-volatile and will not change until it is affected by an externalmagnetic field.

Storing or writing data into a magnetoresistive memory element 10 isaccomplished by applying magnetic fields and thereby causing magneticmaterial in the free layer 12 to be magnetised into either of twopossible memory states. When both magnetic layers 11, 12 of the layeredstructure of a magnetoresistive memory element 10 are magnetised withthe same orientation (parallel), the data is either of two binaryvalues, e.g. “0”, otherwise, if both magnetic layers 11, 12 of thelayered structure of the magnetoresistive memory element 10 aremagnetised with inverse orientation (anti-parallel), the data is theother binary value, e.g. “1”. The magnetic fields are created by passingcurrents through current lines (word lines 14, 14 a, 14 b, 14 c and bitlines 15, 15 a, 15 b, 15 c) external to the magnetic structures. It isto be noted that two magnetic field components are used to differentiatebetween a memory element 10 s and other non-selected memory elements 10.

Reading data is accomplished by sensing resistance changes in a magneticmemory element 10 when magnetic fields are applied. Making use of thefact that the resistance of the layered structure 11, 12, 13 variesdepending on whether or not the magnetic orientations are parallel, thesystem can discriminate both binary values of the data, e.g. “0” or “1”.The magnetic fields required for readout are created by passing currentsthrough word lines external to the magnetic structures, or through themagnetic structures themselves (via bit line 15 and sense lines 16).Reading of a selected memory element 10 s is done through a seriestransistor 17 connected to a via 21 to avoid sneak currents throughother memory elements 10.

The most common MRAM design is the type 1T1MTJ (1 transistor 17 per 1MTJ cell 10), as illustrated in FIG. 1. A memory array 20 comprising aplurality of magnetoresistive memory elements 10 comprises orthogonalbit lines 15 a, 15 b, 15 c and word lines 14 a, 14 b, 14 c patternedseparately into two metal layers respectively under and above themagnetoresistive memory elements 10, in the present example magnetictunnel junction (MTJ) stacks. The bit lines 15 a, 15 b, 15 c areparallel with the hard axis of the memory elements 10, which creates afield in the easy axis, while the word lines 14 a, 14 b, 14 c otherwisecreate a field in the hard axis. In some designs the relations can bereversed, i.e. the bit lines 15 may create a hard axis field and theword lines 14 may create an easy axis field. Writing on a selectedmemory element 10 s is done by simultaneously applying current pulsesthrough the respective bit line 15 b and word line 14 a that intersectat the memory element 10 s. The direction of the resultant field makesan angle of 45° with respect to the easy axis of the free layer 12 ofthe memory element 10 s. At this angle, the switching field of the freelayer 12 is the smallest, thus writing can be done with the leastcurrent.

The switching curve of an MRAM element can be represented by itsso-called astroid curve 30, 31 as shown in FIG. 3. The astroid curves30, 31 unambiguously separate switching and non-switching events fordifferent time periods. Astroid curve 30 is a curve illustrating 10 yearstability for non-selected memory elements 10, and astroid curve 31 is acurve illustrating 10 ns stability for non-selected memory elements 10.In other words, if a magnetic field is being applied within the astroid30, 31, magnetic memory elements 10 will not switch and maintain intheir state for 10 years, respectively 10 ns, whereas fields exceedingthe astroid 30, 31 may switch the memory element 10, if the previousstate were the opposite one. Therefore, only if two magnetic fieldcomponents are present, the bit state of a memory element 10 can beswitched.

If the magnitudes of the magnetic fields generated by either word line14 or bit line 15 are the same, the direction of the resultant magneticfield makes an angle of 45° with respect to the easy axis of the freelayer 12 of the selected element 10 s. At this angle, the switchingfield of the free layer 12 is the smallest, as shown by the astroidcurve 30, 31 in FIG. 3, thus writing can be done with the least current.

On the one hand, the currents in the selected bit line 15 b and wordline 14 a must be chosen in such a way that the total magnetic fieldsufficiently exceeds the switching field of the selected memory element10 s at 45° with the easy axis, or in other words, so that the end ofthe resultant field vector 32 is on or outside the astroid 30, 31 inthis direction (see FIG. 3). On the other hand, the magnitude of thefield created by the selected bit line 15 b must be significantlysmaller than the switching fields in the easy axis direction EA of anyof the memory elements 10 lying on the same bit line 15 b to preventundesired over-writing. Also, the magnitude of the field created by theselected word line 14 a must be significantly smaller than the switchingfields in the hard axis direction HA of any of the memory elements 10lying on the same word line 14 a to prevent undesired over-writing.

FIG. 3 also illustrates stable write field windows 33, i.e. if aresultant magnetic field vector 32, obtained by applying a first currentthrough a selected bit line 15 and a second current through a selectedword line 14, falls within such write field window 33, it may switch themagnetic state of the selected memory element 10 s if the previous statewere the opposite one, but non-selected memory elements 10 located alongone of the selected word or bit lines 14, 15 will not switch states.

For arrays 20 of memory elements 10, a statistical interpretation isgiven to the astroid curve 30, 31. In other words, a standard deviationparameter σ can be assigned to the astroid curve 30, 31 which representsa Gaussian-like switching field distribution.

A one-dimensional representation of the switching field distribution isas represented in FIG. 4. As an example, the distribution of logic ‘0’values is shown on the left hand side of the drawing in graph 40, andthe distribution of logic ‘1’ values is shown on the right hand side ofthe drawing in graph 41. Both distributions have a standard deviationparameter σ, which for simplicity is assumed the same for both logicvalues. It is however to be noted that, in practice, the standarddeviation parameter σ does not need to be the same for bothdistributions.

It is a disadvantage of MRAM cells, and of magnetic memories in general,that an intentional or unintentional exposure to strong magnetic fieldsmakes them vulnerable. Very high density MRAM arrays 20 are particularlysensitive to magnetic fields mainly because the minusculemagnetoresistive memory elements 10 require relatively low magneticfields for read/write operations which depend upon the switching orsensing of magnetic vectors in the free layers 12. These magneticvectors are, in turn, easily affected and may have their magneticorientation changed by such external magnetic fields. The dashed lines42 in FIG. 4 visualise the maximum field range that would be allowed forbit stability, in view of the switching field distribution of the logic‘0’ and ‘1’ values. In other words: if the maximum external fieldexceeds these limits, at least some of the data stored in the memoryelements may be changed.

A solution would be to shield the memory elements from any externalfield. However, also shielding has its limits so that, always, a highermagnetic field can be applied which will cause an external magneticfield in the vicinity of the data layer, and which will have potentialconsequences with respect to data retention.

Therefore, it would be desirable to detect whether a certain magneticfield threshold is exceeded, for which data integrity in the magneticmemory array can not be guaranteed.

In order to solve this problem, a specific magnetic field sensor can bedesigned and integrated on-chip, which sensor is able to detect anymagnetic field threshold. However, adding this functionality would leadto extra costs, e.g. due to extra mask steps during manufacturing of thememory device.

It is an object of the present invention to provide a magnetic fieldsensor for indicating data retention of an array of magnetic memoryelements, without substantially introducing extra costs with respect tothe manufacturing costs of the memory array.

The above objective is accomplished by a method and device according tothe present invention.

The present invention provides an array of magnetoresistive memoryelements provided with at least one data retention indicator device. Theat least one data retention indicator device comprises a first magneticelement and a second magnetic element each having a pre-setmagnetisation direction, the preset magnetisation direction of the firstand second magnetic elements being different from each other. The firstand second magnetic elements are suitable for aligning theirmagnetisation direction with magnetic field lines of an externallyapplied magnetic field exceeding a detection threshold value. Accordingto the present invention, a parameter of the at least one data retentionindicator device is chosen so as to set the detection threshold value ofthe externally applied magnetic field to be detected. The at least onedata retention indicator device has a state or an output indicative ofexposure of the magnetoresistive memory elements of the array to saidexternally applied magnetic field.

The parameter may comprise the geometry of the device, i.e. it mayinclude any or a combination of the shape, size and aspect ratio of thefirst and second magnetic elements.

The first and second magnetic elements may comprise MRAM cells. The MRAMcells have a free magnetic layer, and according to the present inventionthe MRAM cells may have pre-set inverse magnetisation directions oftheir free magnetic layer.

The at least one data retention indicator device may be built adjacentto the magnetic memory elements of which the data retention has to beindicated. A plurality of data retention indicator devices may bespatially distributed amongst the magnetic memory element in the array.

The present invention also provides an integrated circuit comprising anarray of magnetic memory elements according to the present invention.The integrated circuit may furthermore comprise a control circuit forgenerating an error signal upon indication by any of the at least onedata retention indicator devices of exposure of the array to anexternally applied magnetic field exceeding the detection thresholdvalue.

The present invention furthermore provides a method for indicating dataretention of an array of magnetic memory elements. The method compriseschanging a preset magnetisation direction of a magnetic data retentionindicator device when the array is exposed to an external magnetic fieldexceeding a pre-set magnetic field threshold voltage.

These and other characteristics, features and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention. The reference figures quoted below refer to the attacheddrawings.

FIG. 1(a) illustrates the MRAM write principle and FIG. 1(b) illustratesthe MRAM read principle.

FIG. 2 is a perspective view of a known 1T1MTJ MRAM design comprising aplurality of memory elements and perpendicular bit lines and word lines.Magnetic tunnel junctions (MTJ) are placed at the intersection regionsof the bit lines and word lines. The bottom electrodes of the MTJs areconnected to selection transistors with vias, which are used whenreading the memory elements.

FIG. 3 illustrates an astroid curve showing criteria for robust writeoperation in MRAM, resulting in stable write field windows.

FIG. 4 illustrates magnetic field distribution for an array ofmagnetoresistive memory elements, with representative standard deviationparameter σ.

FIG. 5 is a schematic illustration of a side view of a data retentionindicator device according to an embodiment of the present invention.

FIG. 6 shows switching fields of MRAM elements, taking into accountprocess variations.

In the different figures, the same reference figures refer to the sameor analogous elements.

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps. Where an indefinite or definite article is used when referring toa singular noun e.g. “a” or “an”, “the”, this includes a plural of thatnoun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

According to the present invention, an array (not represented in thedrawings) of magnetic memory elements having a data content is provided,which array is provided what least one data retention indicator device50 comprising a first magnetic element 51 and a second magnetic element52.

The array of magnetoresistive memory elements is logically organised inrows and columns. Throughout this description, the terms “horizontal”and “vertical” are used to provide a co-ordinate system and for use ofexplanation only. They do not need to, but may, refer to an actualphysical direction of the device. Furthermore, the terms “row” and“column” are used to describe sets of array elements which are linkedtogether. The linking can be in the form of a Cartesian array of rowsand columns; however, the present invention is not limited thereto. Aswill be understood by those skilled in the art, columns and rows can beeasily interchanged and it is intended in this disclosure that theseterms will be interchangeable. Also, non-Cartesian arrays may beconstructed and are included within the scope of the invention.Accordingly the terms “row” and “column” should be interpreted widely.To facilitate in this wide interpretation, the terminology “logicallyorganised in rows and columns” is used. By this is meant that sets ofmemory elements are linked together in a topologically linearintersecting manner; however, that the physical or topographicalarrangement need not be so. For example, the rows may be circles and thecolumns may be radii of these circles and the circles and radii aredescribed in this invention as “logically organised” in rows andcolumns. Also, specific names of the various lines, e.g. bit line andword line, or row line and column line, are intended to be generic namesused to facilitate the explanation and to refer to a particular functionand this specific choice of words is not intended in any way to limitthe invention. It should be understood that all these terms are usedonly to facilitate a better understanding of the specific structurebeing described, and are in no way intended to limit the invention.

According to the present invention, the data retention indicator device50 comprises two inversely magnetised or biased magnetic elements 51,52. According to a first embodiment of the present invention, theseinversely binary magnetised or biased magnetic elements 51, 52 may beformed by discrete magnetic elements, e.g. by a combination of twoassociated MRAM cells with inversely or differently polarisedmagnetisation directions as shown in FIG. 5. With inverse magnetizationdirections is meant that each element 51, 52 has at least two stable orquasi-stable magnetization directions, and the two elements 51, 52 arearranged so that the magnetization direction of one element 51 isreversed with respect to the other element 52. The magnetizationdirection of each element 51, 52 can be set independently, or bothmagnetization directions can be set in one go. For example, the freelayers 12 of the associated MRAM elements 51, 52 are inverselypolarized, i.e. if the free layer 12 of the fist MRAM element 51 ispolarized in one direction, the free layer 12 of the second MRAM element52 is polarized in an opposite direction. For example, the first MRAMelement 51 may have parallel magnetization directions of its pinnedlayer 11 and its free layer 12 and the second MRAM element 52 may haveanti-parallel magnetization directions of its pinned layer 11 and freelayer 12, as shown in FIG. 5. The pinned layers 11 of the associatedMRAM elements 51, 52 both have the same magnetization direction in theexample represented, while the free layers 12 have differentmagnetization directions. Alternatively, pinned layers 11 of theassociated MRAM elements 51, 52 may have opposite magnetizationdirections (not represented in the drawings). As, according to thepresent embodiment, the free layers 12 of the associated MRAM elementsalso have inverse magnetization directions with respect to each other,either the pinned layer 11 and the free layer 12 of both the first andsecond MRAM element 51, 52 may be inversely polarized or the pinnedlayer 11 and the free layer 12 of both the first and second MRAM element51, 52 may have a parallel polarization.

A data retention indicator device 50 according to the present inventionmay be incorporated in an array of MRAM elements adjacent the MRAMelements of which data retention has to be indicated.

Because of the small dimensions of the MRAM elements in an array and thehigh density thereof, exposure to an external magnetic field may resultin changes to the magnetization directions of the elements. Inparticular the cell-pair-wise opposed magnetization directions of theMRAM elements are disturbed and a parallel orientation of themagnetization directions of free layers 12 of neighboring MRAM elementsis produced. This parallel orientation may be along the direction of theexternal magnetic field, when present. The remanent state will be aparallel orientation of the magnetization directions of free layers 12of neighboring MRAM elements. The data retention indicator device 50 asdescribed in accordance with an embodiment of the present invention, isbased on the above described principle. However, the present inventionis not limited to this embodiment.

When the data retention indicator device 50 is exposed to an externalmagnetic field that is large enough to trigger the device, themagnetization direction of the magnetic elements 51, 52 of the dataretention indicator device 50 will all be changed so as to point moreinto the direction of the externally applied magnetic field. Theeffective change of the magnetization direction of the magnetic elements51, 52 gradually increases with increasing external magnetic field.According to the first embodiment of the present invention, themagnetization direction of the free layer 12 of one of the MRAM elements51, 52 of the data retention indicator device 50 will be changedpermanently, if the externally applied magnetic field exceeds a pre-setthreshold value, for which threshold value particular magnetic elements51, 52 of the data retention indicator device 50 are manufactured.Magnetization vectors of the free layers 12 will all point in the samedirection, when the external magnetic field is no longer present.Through this, both LAM elements 51, 52 of the data retention indicatordevice 50 now have parallel magnetization directions.

It is known that parallel and anti-parallel configurations of MRAMelements 51, 52 have different resistances. The resistance of an MRAMelement 51, 52 is either low or high dependent on the relativepolarization, parallel or anti-parallel, of the free layer 12 withrespect to the pinned magnetic layer 11. Therefore, by measuring theresistance difference of both MRAM elements 51, 52 of the data retentionindicator device 50, their mutual magnetization direction can easily bedetermined. For the embodiment shown in FIG. 5, a significant resistancedifference indicates a normal situation, i.e. one of the MRAM elements51 being in parallel configuration and the other MRAM element 52 beingin anti-parallel configuration, which means the array of MRAM elementshas not been exposed to an external magnetic field exceeding thethreshold value for which the MRAM elements 51, 52 are made. Nosubstantial resistance difference between both MRAM elements 51, 52 ofthe data retention indicator device 50 indicates that both MRAM elements51, 52 have equally polarized magnetization directions and thus has beenexposed to an external magnetic field, which has changed themagnetization direction in the free layer 12 of at least one of the MRAMelements 51, 52 of the data retention indicator device 50.Alternatively, according to any of the embodiments, not represented inthe drawings, with inversely polarized pinned magnetic layers, noresistance difference between both MRAM-cells of the data retentionindicator device indicates a normal situation, and a significantresistance difference between both MRAM-cells of the data retentionindicator device indicates that the data retention indicator device, andthus at least part of the memory array has been exposed to an externalmagnetic field exceeding a threshold value.

In this way, by determining the polarization directions of both magneticelements of the data retention indicator device 50, it can be detectedwhether the magnetic memory array has been under the influence of a toolarge external magnetic field, which results in data retention of themagnetic memory not being guaranteed.

An IC wherein at least one data retention indicator device 50 accordingto the present invention is present, can regularly check thepolarization direction, e.g. the resistance, of the magnetic elements51, 52 of the data retention indicator device 50 during operation. Upondetection of a same polarisation direction for the free layers 12 ofboth magnetic elements 51, 52 of the data retention indicator device 50,e.g. by measuring a resistance difference or a same resistance of twoMRAM elements 51, 52, depending on the configuration thereof, and henceupon detection of exposure to an external magnetic field exceeding athreshold value, the IC can, dependent on what is desirable for thespecific application, erase the data of all MRAM elements of the array,or can reset itself or block its function.

In a further embodiment of the present invention a number of dataretention indicator devices 50 according to the present invention arespatially distributed amongst the MRAM elements of the array.

The present invention provides a data retention indicator device 50 todetect exposure to external magnetic fields that exceed a thresholdvalue, which data retention indicator device 50 can easily be added toan embedded or stand-alone MRAM array. Especially in applications whereintegrity of data is crucial, e.g. program code of the operation systemin embedded MRAM in SoC, the use of a data retention indicator device 50according to the present invention may be of importance. Moreover, itprovides a detection for unintentional exposure to an external magneticfield, e.g. from a permanent magnet or from write equipment for themagnetic strip on a smartcard. The invention can also reduce the needfor implementing very good magnetic shielding in MRAM ICs, sinceunintentional exposure to a large field, which is rare in normal use,can now be detected.

The threshold magnetic field as from which data retention cannot beguaranteed can be tuned by proper choice of geometry of the magneticelements 51, 52 of the data retention indicator device 50. In the idealcase, the astroid curve of each of the magnetic elements 51, 52 ismainly set by two parameters: the shape anisotropy, and the totalcoupling field from pinned layer 11 to free layer 12.

For small magnetic tunnel junctions, as typically used today, thedetermining anisotropy term in the energy balance is set by the shape,leading to an anisotropy field H_(K), representative for the size of theastroid curve. In the ideal case, the astroid curve will reach thereference easy and hard axis at the field H_(K) (FIG. 3).

In essence, the shape anisotropy is inversely proportional to theminimal dimension w (width) and proportional the aspect ratio AR of thedevice (AR=1/w, 1=length). Also the shape itself of the magnetic element51, 52 has some influence, e.g. elliptic or diamond shape elements. Asthis can be implemented for any shape, shape will not be consideredexplicitly in the present disclosure. It lies within the skills of aperson skilled in the art to calculate the anisotropy field for anyshape.

For an ellipse, the anisotropy field H_(K) is found to beH_(K)=4π(t.M) (η _(y)−η_(x))/w,with (t.M) the product of free layer 12 thickness t and saturationmagnetization M, and (η_(y)−η_(x)) a monotonously increasing function ofthe aspect ratio AR=1/w with value 0 for 1/w being 1 (circular) and withvalue 1 for 1/w being ∞.

The coupling field between pinned layer 11 and free layer 12 may lead toa shift of the astroid curve 31 along the easy axis EA. Ideally, thisshift is not present, so that the switching fields for ‘0’ and ‘1’ aresymmetrical with respect to the origin (FIG. 4). The coupling field isthe sum of two different components: Néel coupling, and magneticstray-field coupling. The Néel coupling does not depend on the geometry,and is therefore fixed. The magnetic stray-field coupling however isdefined by the geometry as the magnetic stray-field coupling H˜w^(α)/1,with α approximately equal to 0.2.

In order to design 1^(st) and 2^(nd) magnetic elements 51, 52 of thedata retention device 50 with a pre-set threshold magnetic field value,the method described hereinafter can be used. As an example, it isdesired to detect any magnetic field applied to the memory array, whichexceeds a threshold value of 6.σ below the average switching field ofthe MRAM elements in the array. For a Gaussian distribution, this wouldmean that only 1 in 10⁹ elements would switch under this limit.Depending on the effective switching field distribution, this field canbe calculated, and used as target threshold field value for the dataretention indicator devices 50. Also for the data retention indicatordevice 50, a statistical distribution in the switching threshold can betaken into account

The proper choice of the geometry, primarily given by the length 1 andwidth w, can then be found by mapping the astroid curve that is mainlydefined by the shape anisotropy of the magnetic elements 51, 52 on therequired threshold curve. At the same time, one tries to obey thesymmetry of the astroid curve by cancellation of the Néel coupling bythe stray-field coupling, for a given magnetic tunnel junction stackfrom the memory cells in the MRAM array. The first criterion is the mostimportant. The latter one may be more relaxed and may lead to a slightasymmetry in the astroid curve of the data retention indicator device50. As long as the asymmetry is smaller than other margins such causedby e.g. process parameters, this may be allowed.

For clarification purposes only, the following numerical example isgiven. In a magnetoresistive element, the free layer 12 is a NiFe layerwith a thickness of 5 nm, and the pinned layer is an artificialanti-ferromagnetic stack (AAF) comprising IrMn/CoFe/Ru/CoFe with arespective thickness of the CoFe layers of 3 nm and 2.65 nm. Asindicated, one of the CoFe layers is pinned to a naturalantiferromagnetic layer of IrMn. The size of the elements in themagnetoresistive memory array is chosen to be 200×100 nm².

The magnetic multilayer is described with N magnetic layers using amodified Stoner-Wohlfarth approach, where each layer is denoted by amagnetization angle θ_(i), as well as parameters such as layerthickness, saturation magnetization, magneto-crystalline anisotropy,size and geometry. The interlayer coupling between adjacentferromagnetic layers can be represented by an interfacial energy J_(i).The total energy of the system can be calculated as the sum of differentenergy terms for the different layers, magnetocrystalline anisotropyE_(A), external Zeeman energy E_(H), and demagnetization energy E_(S),plus the coupling energy at the different interfaces.$E_{Tot} = {{\sum\limits_{i = 1}^{N}\left( {E_{A} + E_{H} + E_{S}} \right)} + {\sum\limits_{i = 1}^{N - 1}E_{I}}}$In static equilibrium calculations, the set of magnetization directionsof the different layers can be found as (local) minima after solving theN-dimensional optimization problem. The equilibrium configurations aregiven by the set of N coupled equations$\frac{\partial E_{tot}}{\partial\theta_{i}} = {{0\quad i} = {1\quad\ldots\quad N}}$with the stability condition that all the eigenvalues of the matrix Mshould be positive:$M_{ij} = \frac{\partial^{2}E_{tot}}{{\partial\theta_{i}}{\partial\theta_{j}}}$Typical values for the coupling are as follows:

IrMn—CoFe −0.25 mJ/m²

CoFe—CoFe −0.90 mJ/m²

CoFe—NiFe +0.01 mJ/m²

From a calculation, the switching field for the ideal case can be found,i.e. for σ=0, or thus in case there would not be any process variations,which switching field can be labeled as the so-called ‘average’switching field taking into account process variations. The averageswitching field is shown in FIG. 6 by the curve 60. A certain value forσ is assumed, which can be deduced from experimental data, measurementson a plurality of memory elements processed with the same processing. Inthis example, it is assumed that, inside the astroid curve 60, the 6.σcriterion is shown as the curve 61, which corresponds to, as can befound from a calculation with different geometry, the expected curve fora device of 280×150 nm². It can thus be seen what happens if thegeometry is being changed to largers elements, with slightly smalleraspect ratio. Taking into account the possible influence of processvariations on the data retention indicator device 50 itself, one may optto scale a little further, e.g. to 300×160 nm², or 320×180 nm². It is tobe noted that the aspect ratio decreases with the size of the detectiondevice in order to keep the symmetry in this detection device.

Any external magnetic field that may cause a reversal in one of the bitsstored in a magnetic memory element of the array will cause switching inat least one of the pre-set magnetic detection elements 51, 52 of thedata retention indicator device 50, as long as both alignments are partof this set of elements 51, 52. Moreover this information will beremembered by these elements 51, 52, as they keep their memory statewithout having to apply any external power.

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices according to the present invention, variouschanges or modifications in form and detail may be made withoutdeparting from the scope and spirit of this invention.

1. An array (20) of magnetoresistive memory elements (10) provided withat least one data retention indicator device (50) comprising a firstmagnetic element (51) and a second magnetic element (52) each having apre-set magnetisation direction, the pre-set magnetisation direction ofthe first and second magnetic elements (51, 52) being different fromeach other, the first and second magnetic elements (51, 52) beingsuitable for aligning their magnetisation direction with magnetic fieldlines of an externally applied magnetic field exceeding a detectionthreshold value, wherein a parameter of the at least one data retentionindicator device (50) is chosen so as to set the detection thresholdvalue of the externally applied magnetic field to be detected, the atleast one data retention indicator device (50) having a state or anoutput indicative of exposure of the magnetoresistive memory elements(10) of the array (20) to said externally applied magnetic field.
 2. Anarray (20) according to claim 1, wherein the parameter includes any or acombination of the shape, size and aspect ratio of the first and secondmagnetic elements (51, 52).
 3. An array (20) according to claim 1,wherein the first and second magnetic elements (51, 52) comprise MRAMcells.
 4. An array (20) according to claim 3, the MRAM cells having afree magnetic layer (12), wherein the MRAM cells have pre-set inversemagnetisation directions of their free magnetic layer (12).
 5. An array(20) according to claim 1, wherein the at least one data retentionindicator device (50) is built adjacent to the magnetic memory elements(10) of which the data retention has to be indicated.
 6. An array (20)according to claim 1, there being a plurality of data retentionindicator devices (50) spatially distributed amongst the magnetic memoryelement in the array (20).
 7. An integrated circuit comprising an array(20) of magnetic memory elements (10) according to claim
 1. 8. Anintegrated circuit according to claim 7, furthermore comprising acontrol circuit for generating an error signal upon indication by any ofthe at least one data retention indicator devices (50) of exposure ofthe array to an externally applied magnetic field exceeding thedetection threshold value.
 9. A method for indicating data retention ofan array (20) of magnetic memory elements (10), the method comprisingchanging a pre-set magnetisation direction of a magnetic data retentionindicator device when the array is exposed to an external magnetic fieldexceeding a pre-set magnetic field threshold voltage.